Apparatus for determining melting points



Aug- 17, 1965 c. L. MGDANIEL. 3,200,635

APPARATUS FOR DETERMINING MELTING POINTS Filed June 26, 1963 X-Y RECORDER l? N6 0E INVENTOR F .ez/ /MEU/NG POM/ Clyde L. Mccmzel 65 i I .Wijf BY United States Patent O This invention relates to apparatus for determining melting points, and more particularly to apparatus for determining melting points by electrical resistance measurernents.

A Well-known technique for determining the melting point in a material being heated to known temperatures consists of continuously measuring the electrical resistance of the material. The melting point of the material is that temperature at which the electrical resistance of the material exhibits a marked change or breakin value. To employ this technique, it is necessary to provide a small, uniform sample of the material, and to connect a pair of electrodes to the sample. Heretofore it has been common to construct the small sample in the form of a pellet, disk, or the like, and to either embed the electrodes in the sample, or spring-load the electrodes against the sample. Such an arrangement has several disadvantages. The construction of each sample requires elaborate procedures to insure that the sample is uniform. After a sample is tested, it usually cannot be checked or otherwise' re-used without being reformed. ln addition, the construction and maintenance of the electrodes involve considerable time and expense.

VThe disadvantages of the prior art arrangement of the sample and electrodes are obviated by the present invention, wherein the material whose melting point is to be determined is utilized in any convenient quantity, rather than in small, pellet-sized amounts. Briey, in accordance with the present invention there is provided a crucible or other container for receiving a convenient quantity of the material to be tested. The Crucible is disposed in a suitable furnace to melt the material, and a novel electrode of the present invention is introduced into the melted material. This novel electrode is hollow, andcontains passageways for directing a coolant against the tip thereof, and additionally contains means for measuring the temperature of the tip. The electrode is inserted into the melted material, and then is lifted slightly above the surface of the material until a small amount of material clings between the tip of the electrode and the surface of the material. This small, cone-shaped amount of material, held in place by surface tension, is the samg ple utilized in the present invention.

To determine the melting point of this sample, the tip Accordingly, it is an object of this invention to provide melting point apparatus which utilizes any desired, convenient quantity of the material to be tested.

of the electrode is cooled until the sample freezes. Freez- 'ing is determined by measuring the electrical resistance 'between the electrode and body of molten material, a 'sharp change in resistance being the indication of freez- Another object of this invention is to provide melting point apparatus wherein the sample can readily be checked by performing another melting transition without reforming the sample. A

Still another object is to provide apparatus for determining melting and/or freezing points by means of a novel electrode capable of being cooled to etiect the necessary temperature change.

A further object of this invention is to provide melting point apparatus which is capable of precisely determining, by means of resistance measurements, the (liquidus) temperature at which melting is complete for a material which melts over a range of temperatures.

These and other objects and advantages and features of the present invention will become apparent as the following description is read in conjunction with the accompanying drawing, wherein:

FIG. l is an elevational view, partly in section, of an apparatus embodying the principles of the present invention;

FIG. 2 is a top plan View, on an enlarged scale, of the electrode utilized in the apparatus of FIG. l;

FIGS. 3 and 4 are axial sectional views of the electrode taken on the lines 3 3 and tl-4, respectively, of FIG. 2; and

FlG. 5 is a typical diagram of the electrical resistance versus temperature curves obtained by the apparatus of Fre. 1.

ln the melting point apparatus l0 illustrated in FIG. l, the reference numeral 1l designates any desired, convenient quantity of material whose melting point is to be determined. For most materials, a convenient volume of the material 1l is about a few cubic centimeters, which insures that the material l1 is representative of the source thereof. The material 11 is disposed in a container 12 'that is capable of holding the material in the molten state. The preferred container is a crucible of electrically-conductive material such as platinum, graphite, or the like, so that the container can be used as an electrode connected to the material l1, as will be explained hereinafter.

The container l2 and material il are disposed in a VWire-wound resistance furnace 13 adapted to melt the material il. The furnace 13 has an insulating cover 14 with a central opening through which an elongated electrode l5 projects. This electrode 15 is supported by an adjustable electrically-insulative clamp lo attached to the end of an arm l? carried by a lead-screw arrangement 20. The lead-screw arrangement 2li in turn is supported by a vertical column 2l. secured to a base plate 22 upon which the furnace i3 rests. The lead-screw arrangement Ztl is conventional, comprising a screw 2S journaled between a pair of brackets 26, 27 and having a hand knob 23 attached thereto. The arm i7 is threaded to receive the screw 25, whereby actuation of the hand knob 28 causes the arm 17 to traverse a vertical path. As a consequence, the electrode' is moved lengthwise within the container l2.

The electrode 15 is an electrically-conductive tubular member closed at one end to form a generally-rounded tip 3i). As shown in FlGS. 2 4, a cylindrical rod 31 of a 'suitable refractory material is fitted into the interior of the electrode l5. The rod 3l terminates short of the tip 3% so as to provide a small cavity 32 in the electrode adjacent the tip Btl. Axially extending through this rod 3 5l are four passageways or holes 3.., S4, 35, 36 which communicate between the cavity 32 and the exterior of the electrode l5.

The hole 33 is engaged by a flexible tube 39 connected to a coolant source dll. This source td provides a suitable coolant gas such as air, carbon dioxide, or the like, under pressure, whereby coolant is conveyed down the hole 33, through the cavity 32, and up the hole 35 to be discharged into the atmosphere. The remaining two holes 34, 36 contain a pair of electrical leads d3, ifi that are connected to a thermocouple 45 or other electrical temperature measuring device disposed on the bottom of the cavity 32. As shown in FIG. l, the leads d3, 44 of the thermocouple 45 are connected to the X-axis, or independent variable, terminals of an X-Y recorder e7.

To measure the electrical resistance of the material ll, it is necessary to provide two connections to the material. The electrode l5 affords one connection, and as pointed out hereinbefore, the container l2, if electrically-conductive, provides a convenient second' connection to the material l1. This electrode function of the container l2 is illustrated bythe connection of the lead 5l to the container. If the container l2 is not conductive, the lead 5l should be extended into the material il so as to make good contact therewith, as indicated by the dotted line 51. As shown in FIG. 1, the lead 5l, and another lead 52 connected to the electrode l5, are connected in series with a battery 55 and a resistor 56. Any other suitable source of potential may replace the battery 55, and the resistor 56 may be replaced by any other convenient electrical impedance element. The electrodes l2, l5 are also connected by wires 57, 5S to the Y-axis, or dependent variable, terminals of the X, Y recorder 47.

As will readily be appreciated, the voltage produced by the battery 55 is dropped across the series circuit of the resistor 56 and the resistance between the electrodes 12, 15. Consequently, the voltage across each of the series resistances is directly proportional to its resistivevalue. Since the resistor 56 has a fixed resistance value, the voltage dropped across the electrodes l2, is directly proportional lto the resistance of the material ll therebetween. Accordingly, the voltage fed by the wires 57, 58 to the Y-axis terminals of the recorder 47 is a direct measure of the electrical resistance of the material lll.

Generally, when measuring the resistance of the material l1, it is desirable to utilize only a small sample of the material, in order to obtain an optimally sharp and distinct transition between the solid and liquid states. Such a small sample of the material ll is conveniently obtained in the apparatus of the present invention by means of the lengthwise adjustment of the electrode 15 as provided by the lead-screw arrangement 2li. By lifting the tip 30 of the electrode l5 slightly above the surface of the molten material 1.5, a small cone-shaped sample 6d of the material 11 is produced, as shown in FIG. l.V This sample 60 clings to the tip 39 by virtue of the surface tension of the molten material l1.

To obtain the maximally-elongated sample d@ the hand knob 28 of the lead-screw arrangement 2th is actuated to slowly lift the electrode 15 to the point where the surface tension is insufficient to hold `the sample di?, causing the sampled@ to collapse. The collapse of the sample 60 can readily be detected on the X-Y recorder 47, since the separation of the electrode l5 from the material 11 produces a high resistance reading on the Y-aais of the recorder 47. The height of the electrode l5 at which the sample 6d, collapses is .read ona graduated scale 62 affixed to the vertical column 21. The electrode l5 is then lowered into the melted material lll and again raised to a height slightly less than the height at which the sample collapses, thereby providing the longest possible sample 6l?. After the sample 6G is obtained, the apparatus l0 of FIG. l is operated in the following manner. With the cedure outlined above.

furnace 13 adjusted to maintainthe material l1 in the molten state, the coolant source 46B is turnedV on, causing coolant to pass through the cavity 32 at the bottom of the electrode l5 and thereby cool the tip 3d and sample di). The coolant dow is yadjusted to provide sufficient cooling to freeze the .sample dll. As illustrated by the typical curve 65 in FIG. 5, freezing ofthe sample 6() is indicated by a sharp break'or change in the slope of the resistance versus temperature curve produced by the X-Y recorder Specifically, as the temperature of the sample di) is lowered by the coolant, the resistance of the sample rapidly increases Vuntil the sample solidifies, after which the resistance decreases gradually as the temperature isrfurther lowered. i

After it is thus ascertained thatthe sample dll has been frozen, the coolant source d@ is turned 0H, to allow the sample to be melted by the heat supplied by thefurnace l. As the temperature of the sample Ydil increases, the X-Y recorder 457 produces another vresistance versus temperature curve. substantiallyidentical to the freezing curve previously described and illustrated at 65 in FIG. 5. Thatis, the yresistance of the solid sample di? gradually increases as Vthe temperature ofthe sample rises. j When the melting point is reached, the resistance of the sample 6@ suddenly decreases rapidly. The break or change in the slope of the curve 65 of FlG. 5 thus identities the melting point of the material ll. y

As will readily be appreciated, ythe melting point so determined may easily be checked by repeating the pro- Preferably, `the old sample 60 is dispersed into the reservoir of material lll by lowering the electrode l5 into the lcontainer 12 and stirring the molten material ll; For this purpose, small ns (not shown) may be attached tothe lower portion of the electrode l5 so as lto provide'agitation of the ma- 'terial ll upon rotation of the electrode llSwithin the clamp 16. The electrode l5 is then lifted above the material 11 in the manner described above to provide a new sample 'with which to repeat the resistance versus temperature determinati-ons. Y

From the foregoing, it will further be appreciated that thc solid (frozen) samples d@ provided'by the apparatus lil do not change shape upon melting. The reason for this is, of course, that the solid samples 6@ are obtained by freezing the samples formed in the liquid state, whereby the solid samples upon melting merely revert to their original liquid state without any deformation. This feature of the solid samples dll is especially useful in testing materials which melt over a range of temperatures. Such materials begin to melt at a solidus temperature and completely melt at a higher liquidus temperature. In the resistance versus temperature apparatus of the prior art, wherein the samples comprise pellets or disks pressed or molded from solid (frozen) material, the samples tend to flow and break Contact with the electrodes as soon as the solidus is reached, making it impossibleto determine the liquidus. In the apparatus lil of the present invention, however, rit is possible to precisely determine the liquidus as well as the solidus, since there is no change in the shape or electrical contact of the sample during the transition from the solid to Vliquid states.

Although the present invention has .been illustrated by a specific embodiment thereof, it is obvious that many modifications and variations of the invention will be apparent to those skilled in the art. The invention, therefore, is not to be restricted to the precise details of construction shown and described, but is intendedV to cover all modifications coming within the scope of the appended claims. Y

What is claimed is: Y

l. Melting point'apparatus comprising, a container for holding the material to be tested and having means for electrically contacting the material therein, a furnace, said container'being disposed in said furnace, an elongated,

For most materials this melting curve is electrically-conductive electrode closed at one end to form a tip, a cavity within said electrode adjacent said tip, means for passing coolant through said cavity, means for measuring the temperature of said tip, means for moving said electrode lengthwise within said container, and means for measuring the electrical resistance between said electrode and said Contact means of said container.

2. Melting point apparatus as set forth in claim 1, wherein said tip temperature measuring means provides an electrical output, an X-Y recorder, said electrical out- 10 put being connected to said recorder to drive one of the axes thereof, said resistance measuring means being connected to said recorder to drive the other of the axes thereof.

References Cited by the Examiner UNITED STATES PATENTS 3,173,288 3/65 Davis et al 73--17 LOUIS R. PRINCE, Primary Examiner. 

1. MELTING POINT APPARATUS COMPRISING, A CONTAINER FOR HOLDING THE MATERIAL TO BE TESTED AND HAVING MEANS FOR ELECTRICALLY CONTACTING THE MATERIAL THEREIN, A FURNACE, SAID CONTAINER BEING DISPOSED IN SASID FURNACE, AN ELONGATED, ELECTRICALLY-CONDUCTIVE ELECTRODE CLOSED AT ONE END TO FORM A TIP, A CAVITY WITHIN SAID ELECTRODE ADJACENT SAID TIP, MEANS FOR PASSING COOLANT THROUGH SAID CAVITY, MEANS FOR MEASURING THE TEMPERATURE OF SAID TIP, MEANS FOR MOVING SAID ELECTRODE LENGTHWISE WITHIN SAID CONTAINER, AND MEANS FOR MEASURING THE ELECTRICAL RESISTANCE BETWEEN SAID ELECTRODE AND SAID CONTACT MEANS OF SAID CONTAINER. 